Avoid calling ImageConstIterator::GetIndex() iteratively, in transforms and metrics

[N-Dekker]

ImageConstIterator::GetIndex() is relatively expensive, as it computes the index with each call, by doing m_Image->ComputeIndex(m_Offset) internally.

This pull request avoids calling ImageConstIterator::GetIndex(), either by using an iterator "with index" (for example ImageRegionIteratorWithIndex), or by using an iterator "with only index" (ImageRegionConstIteratorWithOnlyIndex), instead.

Doing so should improve the performance of initialization of metrics. Moreover, it specifically improves the performance of the "DistancePreservingRigidityPenalty" and the "TransformRigidityPenalty" metric. It also improves the performance of the "BSplineTransformWithDiffusion" and the "MultiBSplineTransformWithNormal" transform.

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See also:

• Add ComputeIndex() member function to ImageConstIterator InsightSoftwareConsortium/ITK#5787
• discourse.itk.org: Proposing ImageConstIterator::ComputeIndex(), a clearer alternative to GetIndex()

[N-Dekker]

Regarding the use of an iterator "With Index" (versus using an iterator without index), proposed for TransformRigidityPenaltyTerm, ComputeImageExtremaFilter, and MultiBSplineDeformableTransformWithNormal, I observed that ImageRegionConstIteratorWithIndex is much faster than ImageRegionConstIterator (without index), when both the index and the pixel value are retrieved iteratively. I tried the following for a large 2D image:

for (IteratorType it{ image, region }; !it.IsAtEnd(); ++it)
{
    const auto& index = it.GetIndex();
    result += it.Get() + (index[0] + index[1]) % 8;
}

Using VS2026, Release build and ITK v6.0b02, I observed that ImageRegionConstIterator (without index) can take twice as much time as ImageRegionConstIteratorWithIndex.

[N-Dekker]

To my surprise, when only the index is retrieved, both ImageRegionConstIteratorWithIndex and ImageRegionConstIterator appear faster than ImageRegionIndexRange, on a large 2D image. (ImageRegionConstIteratorWithIndex is then almost 3x as fast as ImageRegionConstIterator, and almost 4x as fast as ImageRegionIndexRange.) 🤔

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• ImageRegionIndexRange is much faster with now with ITK pull request PERF: Make IndexRange iterators _much_ faster, by m_Position data member InsightSoftwareConsortium/ITK#5808 commit InsightSoftwareConsortium/ITK@a2fb4bc but it may take a while before this is available in elastix. So for now, let's use ITK's "WithOnlyIndex" iterator in this PR!